Railroad hopper cars are used to transport bulk lading through railway systems. A railroad hopper car typically includes discharge gates located on the underside of the car for unloading the transported materials. Discharge gates typically include one or more sliding door panels that may be selectively moved between open and closed positions to expose or cover an opening in the undercarriage of the car. Typically, an opening and closing drive mechanism shifts a door panel between open and closed positions via a rack or racks fixed to the panel and an operating shaft. The operating shaft carries pinions which engage the racks. The operating shaft is rotated to move the panel in the desired direction. The car may be unloaded by sliding the panel to open the gate and allowing the lading to flow through the opening.
Typically door panels are driven using a rack and pinion system whereby elongated, multitoothed racks are attached along opposing sides of either the upper or lower face of the panel. The rack is engaged by a pinion which is in turn driven by a shaft. The shaft extends outwardly for access and terminates in a socket or other structure that may be engaged by a lever, handle or powered driver used to turn the shaft. Because of its length, as the shaft turns it is subject to undesirable transverse deflection. Additionally, a stop at each end of the rack is engaged by the pinion to limit the travel of the panel, which may cause an impact that can damage the drive components or produce excessive wear over time.
Pinions used in discharge gates are typically circular, external, spur gears having teeth that are either relatively straight-sided or crowned in profile. Crowned tooth profiles include teeth having working surfaces compliant to elliptical, cycloidal, epicycloidal or involute curves. The pitches of the pinion teeth and rack teeth are optimized for engagement with one another. The pitch circle of the pinion ideally bears a relationship to a corresponding pitch line of the rack such that the two pitches will provide a common velocity when the pinion is in rolling contact with the rack.
Lading carried by hopper cars typically includes granular or particulate matter such as sugar, flour, grain, plastic pellets and cement. The weight of the lading in a full hopper car can exert considerable downward force against the door panels causing them to deflect as well as resist movement under load. When the panel deflects, the proximity of the rack to the pinion is altered. As the pinion moves towards or away from the rack the effective pitch of the pinion teeth is changed and thus the rack teeth and pinion teeth may no longer align properly. As a result of this misalignment, the pinion teeth may skip (if the pinion is moved further away from the rack) or climb (if the pinion is moved closer to the rack).
Since discharge gates are subjected to substantial jarring while the hopper car is in use, the door panel should be restrained from opening inadvertently. In particular, the door is subject to inertial forces that would tend to cause it to open when the hopper car is stopped suddenly. To secure the door panel in a closed position, discharge gates are often provided with latches or locks positioned under the panel that may be activated by inertial force during transportation of lading within the hopper car and released prior to opening the discharge gate during unloading.